If someone leaves Islam, either for a short of a long period of time, and later wants to come back and repent, what are the necessary steps?

Should he just repent to Allah (as other sins)? Or should he officially say Shahada and take a bath just after it and do any other necessary steps as someone who converted to Islam from another religion?

And if an apostate does not do any official thing of becoming a Muslim, just repents, has his belief back, and takes a bath daily (not formally saying Shahada and taking a bath right after it), would he be a Muslim and his prayers accepted by Allah?

Is it that once someone becomes a Muslim, he can never leave Islam, but if he disbelieves, it will be a huge sin, but that does not make him a non-Muslim and he does not have to become a Muslim again, but only needs to repent?

Say, "O My servants who have transgressed against themselves [by
sinning], do not despair of the mercy of Allah. Indeed, Allah forgives
all sins. Indeed, it is He who is the Forgiving, the Merciful."
-- 39:53

Becoming Muslim does not need anything official. Religion is belief and belief is something in mind. Everything happens in mind. Allah looks at your mind. Not your official claims.